Cylinder pressure is an important control parameter for closed loop control of combustion engines. Existing cylinder pressure sensors, typically based on either piezoelectric or piezoresistive measurement, suffer from poor reliability and longevity. In many cases the operational life time of existing cylinder pressure sensors is a tenth of the expected time between major engine overhaul. The poor reliability and longevity of current sensors causes drastic increases in maintenance costs and increases out of service times for engines. As an example, existing cylinder pressure sensors have a typical lifetime of about 2000 hours and the target for marine applications of such sensors is 16000 hours.
Typically cylinder pressure sensors of the piezoresistive varieties include a strain gauge which is attached to a bending membrane. The bending membrane bends the gauge and the electrical resistivity of the sensor gauge changes. This change in the electrical resistivity can be measured. Piezoelectrical sensors include a piezoelectrical ceramic which is compressed with pretension in between a moving membrane and a rigid support. Applying pressure to the membrane causes an electrical charge which can be measured.
Typically, in existing sensor designs, the measuring element is in direct contact with a hot deflecting membrane. This causes high mechanical and thermal stresses to the measuring element. Therefore there is a need for thermal insulation between the sensing element and the membrane. Such implementations are difficult and often lead to difficult mechanical solutions. In the presented solution these problems are solved and a simple and reliable pressure sensor can be implemented.